The properties of glass make it an ideal substrate material for use in many applications. In particular, the combination of optical transparency, with reasonable strength at a nominal cost, allows the widespread use of glass products. Glass, however, does suffer from several limitations. Glass is not a particularly hard material, and consequently it abrades in many applications. Additionally, glass is chemically reactive with many alkaline substances and with hydrofluoric acid. New applications and superior performance in existing applications could be thus realized for glass products if glass were more abrasion resistant and less chemically reactive. Examples of glass products which could benefit from improved abrasion resistance include eyeglass and sunglass lenses, architectural glass, analytical instrument windows, automotive windshields and laser bar code scanners for use in retail stores and supermarkets.
Diamond-like carbon films (DLC) are well known in the art and have been recognized as potential coatings to enhance the abrasion resistance of various substrate materials, including glass. The DLC coatings possess excellent optical properties and exhibit excellent resistance to abrasion and chemical attack by various acids, including hydrofluoric acid. However, it has been found that the DLC coatings will impart improved abrasion resistance to a substrate only if the adherence of the coating to the parent substrate is excellent.
The most obvious and common approach to coating the glass substrate is to apply the DLC coating directly onto a clean glass surface. However, this approach often results in a DLC coating which displays poor adhesion and therefore, poor abrasion resistance. DLC coatings are typically under significant compressive stress. This stress greatly affects the ability of the coating to remain adherent to the glass substrate. Additionally, glass often contains many alkali oxides and other additives which can inhibit the bonding of the SiO.sub.2 in the glass to the carbon atoms in the DLC coating. It is currently believed that the reaction between the SiO.sub.2 in glass and the DLC is essential for the coating to exhibit excellent adhesion. Therefore, less obvious methods are required to produce a glass substrate with a highly adherent DLC coating which provides excellent abrasion resistance.
In addition to glass substrates, many other optically transparent substrate materials, such as sapphire, glassy-ceramics, salts (NaCl, KBr, KCl, etc.), metal fluorides and metal oxides could benefit from a DLC coating, but contain elements which inhibit the bonding of the DLC layer.
Many methods for depositing DLC have been demonstrated, including radio frequency plasma deposition, ion beam sputter deposition from a carbon target, ion beam sputtered carbon with ion beam assist, direct ion beam deposition, dual ion beam deposition, laser ablation deposition from a carbon target, and ion beam assisted evaporation of carbon. Many of these prior art techniques have been used to deposit DLC on glass substrates, however, the emphasis of the prior art has not been on the adhesion of the DLC to the glass substrate or on the abrasion resistance of the coated substrate product. Illustrative are the following references: U.S. Pat. Nos. 4,746,538; 4,400,410; 4,383,728; 4,504,519; 4,603,082; 4,060,660; 4,877,677; 4,569,738 and 4,661,409; Japanese Patent Nos. 63/221841; 63/221840; 63/195266; 1147068; 1147067; 64-2001; 59-26906 and 51128686; European Patent Nos. DD-203903; SU1006402; European Patent Application #EPO 243541 (WO 87/02713); Deutchman, et al.; Proc. SPIE-Int. Soc. Opt. Eng.1146, 124-34, 1989; Collins, et at., Proc. SPIE-Int. Soc. Opt. Eng. 1146, 37-47, 1989; Liou, et al., Proc. PIE-Int. Soc. Opt. Eng. 1146, 12-20, 1989; Bubenzer, et al., Proc. DARPA Workshop Diamond-Like Carbon Coat., Meeting date 1982, Issue AD-A136 766, 33-47, edited by B. Bendow in NBS Spec. Publ. 669, 249-54, 1984; NBS Spec. Publ. 638, 482-82, 1984; Bubenzer, et al., NBS Spec. Publ. 638, 477-81, 1984; Appl. Phys. Lett. 55, 631-3, 1989; J. Vac. Sci. Technol A7, 2307-10, 1989; and D. Nir, Thin Solid Films, 144, 201-9, 1986. These references do not however describe the use of transparent interlayers to improve the adhesion of the amorphous carbon coating to the substrate or substantially optically transparent DLC coatings with greatly improved wear resistance for severe abrasive environments.